The future demands for high-end computer tomograph (CT) and cardiovascular (CV) imaging regarding the X-ray source are (1) higher power/tube current, (2) smaller focal spots combined with the ability of active control of the size, ratio and position of the focal spot, (3) shorter times for cooling down, and, concerning CT, (4) shorter gantry rotation times. In addition to this, the tube design is limited in length and weight to achieve an easy handling for CV applications and a realisable gantry setup for CT applications.
One key to reach higher power and faster cooling is given by using a sophisticated heat management concept inside the X-ray tube. In conventional bipolar X-ray tubes about 40% of the thermal load of the target is due to electrons backscattered from the target, which are reaccelerated towards the target and hitting it again outside the focal spot. Hence these electrons contribute to the temperature increase of the target and cause off-focal radiation. Therefore one key component of a currently developed new X-ray tube generation is a scattered electron collector (SEC) located in front of the target. Introducing this component in combination with a unipolar tube setup causes an electrical field-free region above the target if both elements—target and SEC—are on the same potential. The thermal load of the target is in this case determined only by electrons contributing to the tube's X-ray output. The backscattered electrons release their energy at the SEC which is integrated into the tube's cooling system.
Conventionally, this setup including a SEC enhances the distance between anode and cathode but leaves no space for focusing elements. Compared to prior X-ray tubes this causes a drastically enlarged electron beam path making the focusing of the electron beam more advanced.
One major goal of new high-end X-ray tubes for medical examinations is to provide variable and small focal spot sizes and positions within a high voltage range of U=60-150 kV and tube currents up to I=2A. Additionally limitations in the tube size with an optical length of 1<130 mm have to be taken into account.
Image quality issues in CT or CV imaging require the possibility of an active control of the focal spot size during image acquisition. New imaging modalities in CT like dynamic focal spot (deflection in tangential and radial direction) which help to increase spatial resolution or to reduce artifacts need in addition the ability of active focal spot position control.
For satisfying the above and other requirements, there may be a need for an improved electron optical apparatus for producing an electron beam, an improved X-ray emitting device and an improved method for producing an electron beam.